Carbon monoxide hydrogenation on chromium sesquioxide-promoted copper catalysts

Abstract

Ids (v 1 0.5 1 .o 1 .s 0 Figure 6. Photocurrent quantum efficiency ( q ) vs plots obtained from the experimental data of Figure 2.the vertical axis gives us a k'\ = 5.0 X and the slope a k12 = 5.0 X cmz cl, very close to k'2 and kN2 values associated with ND values obtained from Mott-Schottky plots.As far as eq 19 and 20 are valid, these results shows that the catalytic activity of Ti203 corundum units associated with oxygen vacancies is characterized by a rate constant for OHo, reaction about 5 orders of magnitude greater than that the characterizing rutile regions free of this kind of lattice defect. ConclusionsWe have shown that the time decay characterizing the photocurrent transients observed during water photoelectrolysis at (001) n-Ti02 rutile in basic medium, and in the absence of surface recombination (high enough band-bending) is very sensitive to the concentration of donor centers ND Under these conditions, the photocurrent-time dependence is due to the surface accumulation of positive holes, the photocurrent-time decay being controlled by rate of generation of (HzOJ, specia from the surface reaction between photogenerated OHo, radicals.'" The experimental observation that the rate constant for (H2Oz)# generation, k2, increases linearly with ND suggests that Ti3+-Vo-Ti3+ lattice defects generated by Ti02 reduction in H2 atmosphere behave as active centers for the photoassisted oxidation of H20 to H20z on the semiconductor surface. The catalytic properties of Ti203 corundum units are attributed to the short distance between adjacent Ti3+ ions (2.59 A), which is considerably smaller than the comsponding Tibc-Tibc distance in the nondefective rutile regions (4.59 A). This assumption is supported by recent calculations of Harding, according to which for 0-0 separations lower than =3.0 A the covalent bonding involved in the formation of H202 peroxo species is found to be the dominant attractive force between two interacting OHo radicals.Acknowledgment. This work was supported by the CICyT, Spain (Project MAT 89-0862-CO3-01). R-try No.The promotion effect of Cr203 on copper catalysts has been studied on Cu/Cr203, Cu/Cr/Si02, and Cu/Si02 catalysts. CO hydrogenation reaction was applied as a probe to test the activity of the catalysts. The activities of Cu/Cr203 catalysts for CO hydrogenation are highly dependent on the variations of the particle size of the supported copper and the temperature of reduction treatments. Cu/Cr/Si02 catalysts have similar behavior as the Cu/Cr203 catalysts. Si02-supportcd copper catalysts are insensitive to the change of pretreatment conditions of the catalysts and the amounts of copper loading on the support. The origin of the promotion ability of Cr203 on copper catalysts was discussed on the basis of an electronic interaction model.